The growing importance of alternative energy sources and issues raised by stranded gas have brought a renewed interest in the Fischer-Tropsch synthesis, which is one of the more attractive direct and environmentally acceptable paths to high quality transportation fuels. Fischer-Tropsch synthesis involves the production of hydrocarbons by the catalyzed reaction of CO and hydrogen. Research involving the Fischer-Tropsch process has been conducted since the 1920's, and commercial plants have operated in Germany, South Africa and other parts of the world based on the use of particular catalysts.
U.S. Pat. No. 4,046,829 to Ireland et al. appears to disclose a process, wherein (in the process as modified) the product of Fischer-Tropsch synthesis is separated to recover a product boiling above and below about 400 degrees F., which is thereafter separately processed over different beds of ZSM-5 crystalline zeolite under conditions promoting the formation of fuel oil products and gasoline of higher octane rating. As disclosed therein, the unmodified process performed a separation of the Fischer-Tropsch synthesis product into various fractions: C2-, C3-C4, gasoline, fuel oil (diesel) and waxy oil.
U.S. Pat. No. 4,088,671 to Kobylinski appears to disclose the use of a ruthenium promoted cobalt catalyst on a support such as alumina or kielsguhr, in the synthesis of hydrocarbons from the reaction of CO and hydrogen at substantially atmospheric pressure. It was found that the addition of small amounts of ruthenium to a cobalt synthesis catalyst resulted in substantial elimination of methane from the product, together with the production of a more saturated, higher average carbon number. Aqueous solutions of metal salts were used to impregnate the support to prepare the catalyst thereof. The C9+ fraction was about 88% by weight, with the C19+ fraction being about 45% by weight. This fraction contains the portion of the synthetic crude, (or syncrude) which is normally solid at ambient temperatures (C20+) and is commonly referred to a wax, which leaves about 43% by weight in the diesel range.
Research was performed to reduce the waxy portion of the diesel fraction to minimize the effects of the wax coating the catalyst and thereby deactivating the catalyst and reducing the efficiency thereof. In one approach, dual catalysts were used in a single stage. U.S. Pat. No. 4,906,671 to Haag et al. appears to disclose a Fischer-Tropsch catalyst used in combination with a zeolite catalyst, wherein the zeolite catalyst selectively converted enough of the waxy product to prevent adhesion between catalyst particles which might interfere with catalyst flow thereby permitting maximization of diesel oil and heavy hydrocarbon yield. The diesel oil yield is disclosed to range from about 15 to about 45% by weight.
U.S. Pat. No. 4,652,538 to Rabo et al. appears to disclose the use of a dual catalyst composition in a single stage, wherein the composition is said to be capable of ensuring the production of only relatively minor amounts of heavy products boiling beyond the diesel oil range. The catalyst composition employed a Fischer-Tropsch catalyst together with a steam-stabilized zeolite Y catalyst of hydrophobic character, desirably in acid extracted form.
In another approach, the composition of the Fischer-Tropsch catalyst was modified to enhance diesel fuel boiling point range product.
U.S. Pat. Nos. 4,413,064 and 4,493,905 to Beuther et al. appear to disclose a catalyst useful in the conversion of synthesis gas to diesel fuel in a fluidized bed. The catalyst is prepared by contacting finely divided alumina with an aqueous impregnation solution of a cobalt salt, drying the impregnated support and thereafter contacting the support with a non-aqueous, organic impregnation solution of salts of ruthenium and a Group IIIB or IVB metal. The diesel fuel fraction (C9-C20) ranged from about 25 to about 57% by weight, with the C21+ fraction ranging from about 1 to about 9% by weight.
U.S. Pat. No. 4,605,680 to Beuther et al. appears to disclose the conversion of synthesis gas to diesel fuel and a high octane gasoline in two stages. In the first stage, the synthesis gas is converted to straight chain paraffins mainly boiling in the diesel fuel range. The diesel range fraction (C9-C20) ranged from about 44 to about 62% by weight, with the C21+ fraction ranging from about 4 to about 9% by weight. This first stage utilizes a catalyst consisting essentially of cobalt, preferably promoted with a Group IIIB or IVB metal oxide, on a support of gamma-alumina, eta-alumina or mixtures thereof. A portion of the straight chain paraffins in the C5-C8 range is separated and then converted in a second stage to a highly aromatic and branched chain paraffinic gasoline using a platinum group metal catalyst.
U.S. Pat. No. 4,613,624 to Beuther et al. appears to disclose the conversion of synthesis gas to straight chain paraffins in the diesel fuel boiling point range. The diesel range fraction ranged from about 33 to about 65% by weight, with the C21+ fraction ranging from nil to about 25% by weight. The catalyst consisted essentially of cobalt and a Group IIIB or IVB metal oxide on an alumina support of gamma-alumina, eta-alumina or mixtures thereof where the catalyst has a hydrogen chemisorption value of between about 100 and about 300 micromol per gram.
U.S. Pat. Nos. 4,568,663 and 4,670,475 to Mauldin appear to disclose a rhenium promoted cobalt catalyst, especially rhenium and thoria promoted cobalt catalyst, used in a process for the conversion of synthesis gas to an admixture of C10+ linear paraffins and olefins. These hydrocarbons can then be refined particularly to premium middle distillate fuels of carbon number ranging from about C10 to about C20. This Fischer-Tropsch synthesis product contains C10+ hydrocarbons in the amount of at least about 60% by weight (Examples thereof disclose about 80+% by weight). However, no distinction is made between the diesel and wax fractions thereof.
Among other things, the foregoing references do not disclose or teach how these hydrocarbons produced via Fischer-Tropsch synthesis would be formulated as a fuel nor how well they would perform.
U.S. Pat. No. 5,506,272 to Benham et al. appears to disclose several Fischer-Tropsch schemes using a promoted iron catalyst in a slurry reactor to produce oxygenated diesel and naphtha fractions on distillation that reduce particulate emissions in diesel engines. The Fischer-Tropsch synthesis product is separated into various fractions: tail gas, C5-C20 hydrocarbon product, water and alcohols, light wax and heavy wax. The C5-C20 product is generally a mixture of saturated and unsaturated aliphatic hydrocarbons. The C5-C20 hydrocarbon product can be employed as a substitute for diesel fuel and the like and hava high cetane numbers (about 62) thereof. The synthetic diesel fuel appeared to contain a distribution of C3-C19 alcohols and other oxygenates as a result of the Fischer-Tropsch synthesis. In one composition, the alcohols and oxygenates were each present in an amount of about 6% by weight. It was further disclosed that the enhanced emissions performance suggested that an oxygen-containing additive could be formulated which would produce improved performance. Additional diesel fuel may be prepared by cracking the wax portion of the Fischer-Tropsch synthesis product. This diesel product had a cetane number of about 73, but a low oxygen content (about 0.16%). The reference discloses that the two types of synthetic diesel produced thereby may be blended to increase the oxygen content of the mixture over the cracked product. The naphtha product thereof appeared to contain several oxygen-containing specie including C8-C12 alcohols (about 30%).
U.S. Pat. No. 5,807,413 to Wittenbrink et al. appears to disclose a synthetic diesel fuel with reduced particulate emissions. The diesel engine fuel is produced from Fischer-Tropsch wax by separating a light density fraction, e.g., C5-C15, preferably C7-C14, having at least 80+% by weight n-paraffins. The fuel composition appears to have comprised (1) predominantly C5-C15 paraffin hydrocarbons of which at least 80% by weight are n-paraffins, (2) no more than 5000 ppm alcohols as oxygen, (3) no more than 10% by weight olefins, (4) no more than 0.05% by weight aromatics, (5) no more than 0.001% by weight sulfur, (6) no more than 0.001% by weight nitrogen and (7) a cetane number of at least 60.
The addition of ethanol or similar blend stocks to petroleum-based diesel has been investigated by several researchers. Unlike mixtures of oxygenates with gasoline, mixtures of oxygenates with diesel appears to have not been accepted as providing performance advantages that justify commercialization.
Eckland et al (SAE Paper 840118) present a "State-of-the-Art Report on the Use of Alcohols in Diesel Engines". Techniques that have been evaluated for concurrent use of petroleum-based diesel and alcohols in a compression-ignition engine include (1) alcohol fumigation, (2) dual injection (3) alcohol/diesel fuel emulsions, and (4) alcohol/diesel fuel solutions.
Fumigation and dual injection require additional and separate fuel handling systems including additional injectors for either manifold injection (for fumigation) or direct injection. Accordingly, these alternatives represent both a significant incremental cost for vehicle production and increased operational inconvenience related to refilling two fuel tanks rather than one.
In the case of fumigation, Heisey and Lestz (SAE Paper 811208) report significant reductions in particulate generation; however, NO.sub.x generation increases. The incremental vehicular costs and increased NO.sub.x associated with fumigation have limited its acceptance.
The prominent embodiments of the present invention do not include fumigation or dual injection.
To maintain stable fuel emulsions of alcohol and diesel, large amounts of costly emulsifiers are required. Baker of the Southwest Research Institute (SAE Paper 810254) reported that 9:10 and 3:2 parts by volume of alcohol to emulsifier were required by methanol and ethanol, respectively to create stable emulsions. Emulsifiers are needed with methanol. They are needed with ethanol when the water content of ethanol is greater than about 0.5%.
Hsu (SAE Paper 860300) reports decreased NO.sub.x and smoke but increased hydrocarbon emissions with diesel-water emulsions. Likos et al (SAE Paper 821039) reports increased NO.sub.x and hydrocarbon emissions for diesel-ethanol emulsions. Khan and Gollahalli (SAE Paper 811210) report decreased NO.sub.x and hydrocarbon emissions with increased particulate emissions for diesel-ethanol emulsions. Lawson et al (SAE Paper 810346) report increased NO.sub.x and decreased particulate emissions with diesel-methanol emulsions.
The prominent embodiments of the present invention are not emulsions and thus have the advantage of not relying on the use of large amounts of expensive emulsifiers or mixing equipment.
Alcohol-diesel fuel solutions form a homogenous phase rather than two liquid phases as with emulsions. Methanol is not soluble in petroleum-based diesel, and so, most solution work has been performed with ethanol. A disadvantage of solutions is that two liquid phases form when the alcohol-diesel mixture is contacted with water. Although this can manifest into operating difficulties, similar problems occur with straight petroleum-based diesel is contacted with water.
Baker of the Southwest Research Institute (SAE Paper 810254) reports diesel-ethanol emulsions produce similar NO.sub.x, hydrocarbon, and particulate emulsions as compared to baseline runs with straight diesel. Khan and Gollahalli (SAE Paper 811210) report increased particulate emissions with ethanol-diesel mixtures. Test results of ethanol-diesel solutions are inconclusive and mixed.
Many experienced automotive engineers associate a direct correlation between increases in alcohol fractions with increases in NO.sub.x, and recognize that the chemically bound oxygen can lead to reductions in particulate emissions at the proper operating conditions. Since NO.sub.x emissions increase, advantages of ethanol-diesel emissions are limited, and such mixtures have not been generally accepted for widespread use by the market.
The prominent embodiments of the present invention are not mixtures with petroleum-based diesel. Furthermore, advantages of preferred mixtures of the present invention provide significant reductions in both NO.sub.x and particulate emissions. The preferred embodiments of this invention may also lead to increased hydrocarbon emissions; however, this is not considered a significant obstacle and such emissions may be reduced through optimization of the diesel fuel composition of the present invention.
Accordingly, there is a need for synthetic diesel fuels having the required physical, chemical and performance properties for use as a transportation fuel in diesel engines.